1. Field of the Invention
The present invention relates to a voltage supply unit for diagnosing electrical disconnections which may occur in a communication system and an apparatus using the voltage supply unit, and in particular, to the voltage supply unit and the apparatus which are dedicated to the communication system equipped with a differential two-wire type of communication line serving as a transmission path (communication bus) composing a network.
2. Description of the Related Art
Recent vehicles are provided with in-vehicle communication systems with which a variety of in-vehicle devices are controlled. In such in-vehicle communication systems, a communication system that uses a differential two-wire type of communication line is familiar, which composes a network in the vehicle.
This type of communication system adopts a network provided with a transmission path generally consisting of a main line and a plurality of branch lines connected to the main line. A node is connected to each branch line. Both ends of the main line are terminated by termination circuits.
In particular, this kind of in-vehicle communication system uses, as a standard communication protocol, CAN (Controller Area Network) regulated by ISO11898-1. When the CAN is provided with a transceiver that operates on ISO11898-2, a disconnection (breaking of wire), if any, occurring in one of the branch lines of in a node causes the node (also referred to as a “faulty node”) to disturb (or interrupt, jam) communication being carried out between other nodes, not limited to a situation where the faulty node cannot communicate with other nodes.
The above problems result from the fact that the faulty node is no longer given a sufficient difference voltage between its terminals connected to the two communication lines. An insufficient voltage difference makes it difficult to detect the dominant state (i.e., a state in communication) of the transmission path, resulting in abnormal signal reception. A slight increase or decrease in the dominant voltage on one of the two communication lines may cause the difference voltage between the communication lines to be lower than the threshold.
When a faulty node cannot receive signals in the normal condition, the faulty node transmits an error frame or starts the transmission even where another node is already engage transmitting. When the faulty node starts transmitting, it is unable to output normal signals. Because of being in an error state or rejected in arbitration, the faulty node will re-transmit repeat resulting in erroneous transmission.
When the faulty node performs such an erroneous transmission, the transmitted signal waveforms are incomplete owing to the fact that only one of the two communication lines is connected to the main line. Such incomplete signal waveforms interfere with signal waveforms transmitted to the transmission line from the other normal nodes, thereby disturbing communication between or among the other normal nodes.
As a result, the communication system itself does not work correctly. For example, a fault diagnosis apparatus, which has been conventionally used for diagnosing vehicle faults, cannot locate the faulty portions of the vehicle. For this reason, it is necessary to disassemble the vehicle to visually check the connection of each node or to check the conduction of each section of the transmission path, consuming time and effort.
To prevent a faulty node from disturbing communication between or among the other nodes, a technique disclosed by Japanese Patent Application Publication (Laid-open) No. 2006-135375 is known. According to this publication, a master node sends out status information at intervals, and the remaining nodes stop transmission unless they receive the status information.
However, the technique disclosed by the above publication has the following problem. That is, when one of the communication lines of the branch line connected to the master node is disconnected (broken or dropped off from the terminal), all the nodes are obliged to stop signal transmission. In addition, when the status information transmitted from the master node is disturbed by the faulty node, one of the communication lines of the branch line of which is disconnected, all the nodes are also forced to stop of transmission of signals. Thus, the effect of a disconnection can spread widely throughout the network.
Another countermeasure against such a disconnection is shown by ISO11898-3, “Road vehicles? Controller area network (CAN)? Part 3; Low-speed fault tolerant medium dependent interface.” This standard provides a technique for utilizing only normal communication lines with no disconnection. However, this technique's communication speed is limited to 125 kbps, thus providing no practical way. Additionally, this technique cannot provide compatibility with an interface on ISO11898-2, which is provided in ISO15765 (Diagnostics on Controller Area Network) which is for diagnosing faults of vehicles, so that the situation is worsened.